Science without sense…double nonsense

Píldoras sobre medicina basada en pruebas

Gathering the gold nuggets

This post is also available in: Spanish


I was thinking about today’s post and I cannot help remembering the gold-seekers of the Alaskan gold rush of the late nineteenth century. They went traveling to Yukon, looking for a good creek like the Bonanza and collecting tons of mud. But that mud was not the last step of the quest. Among the sediments they had to extract the longed gold nuggets, for which they carefully filtered the sediments to keep only the gold, when there was any.

When we look for the best scientific evidence to solve our clinical questions we do something similar. Normally we chose one of the Internet search engines (like Pubmed, our Bonanza Creek) and we usually get a long list of results (our great deal of mud) that, finally, we will have to filter to extract the gold nuggets, if there are any among the search results.

We have already seen in previous posts how to do a simple search (the least specific and which will provide us with more mud) and how to refine the searches by using the MeSH terms or the advanced search form, with which we try to get less mud and more nuggets.

However, the usual situation is that, once we have the list of results, we have to filter it to keep only what interests us most. Well, for that there is a very popular tool within Pubmed that is, oh surprise, the use of filters.

Let’s see an example. Suppose we want to seek information about the relationship between asthma and obesity in childhood. The ideal would be to build a structured clinical question to perform a specific search, but to show more clearly how filters work we will do a simple “bad designed” search with natural language, to obtain a greater number of results.

I open Pubmed’s home page, type asthma and obesity in children in the search box and press the “Search” button. I get 1169 results, although the number may vary if you do the search at another time.

You can see the result in the first figure. If you look closer, in the left margin of the screen there is a list of text with headings such as “Article types”, “text availability”, etc. Each section is one of the filters that I have selected to be shown in my results screen. You see that there are two links below. The first one says “Clear all” and serves to unmark all the filters that we have selected (in this case, still none). The second one says “Show additional filters” and, if we click on it, a screen with all the available filters appears so that we choose which we want them to be displayed on the screen. Take a look at all the possibilities.

When we want to apply a filter, we just have to click on the text under each filter header. In our case we will filter only the clinical trials published in the last five years and of which the full free text is available (without having to pay a subscription). To do this, click on “Clinical Trial”, “Free full text” and “5 years”, as you can see in the second figure. You can see that the list of results has been reduced to 11, a much more manageable figure than the original 1169.

Now we can remove filters one by one (by clicking on the word “clear” next to each filter), remove them all (by clicking “Clear all”) or add new ones (clicking on the filter we want).

Two precautions to take into account with the use of filters. First, filters will remain active until we deactivate them. If we do not realize it and deactivate them, we can apply them to searches that we do later and get fewer results than expected. Second, filters are built using the MeSH terms that have been assigned to each article at the time of indexing, so very recent articles, which has not been indexed yet and, therefore, have not get their MeSH terms allocated, will be lost when applying the filters. That is why it is advisable to apply the filters at the end of the search process, which is better to make more specific using other techniques such as the use of MeSH or advanced search.

Another option we have with indexes is to automate them for all the searches but without reducing the number of results. To do this we have to open an account in Pubmed by clicking on “Sign in to NCBI” in the upper right corner of the screen. Once we use the search engine as a registered user, we can click on a link above to the right that says “Manage filters” and select the filters we want. In the future, the searches that we do will be without filters, but above to the right you will see links to the filters that we have selected with the number of results in parentheses (you can see it in the first two figures that I have shown). By clicking, we will filter the list of results in a similar way as we did with the other filters, which are accessible without registering.

I would not like to leave the topic of Pubmed and its filters without talking about another search resource: Clinical Queries. You can access them by clicking on the “Pubmed Tools” on the home page of the search engine. Clinical Queries are a kind of filter built by Pubmed developers who filter the search so that only articles related to clinical research are shown.

We type the search string in the search box and we obtain the results distributed in three columns, as you see in the third figure attached. In the first column they are sorted according to the type of study (etiology, diagnosis, treatment, prognosis and clinical prediction guidelines) and the scope of the search that may be more specific (“Narrow”) or less (“Broad”). If we select “treatment” and narrow range (“Narrow”), we see that the search is limited to 25 articles.

The second column lists systematic reviews, meta-analyzes, reviews of evidence-based medicine, etc. Finally, the third focuses on papers on genetics.

If we want to see the complete list we can click on “See all” at the bottom of the list. We will then see a screen similar to the results of a simple or advanced search, as you see in the fourth attached figure. If you look at the search box, the search string has been slightly modified. Once we have this list we can modify the search string and press “Search” again, reapply the filters that suit us, etc. As you can see, the possibilities are endless.

And with this I think we’re going to say goodbye to Pubmed. I encourage you to investigate many other options and tools that are explained in the tutorials of the website, some of which will require you to have an account at NCBI (remember it’s free). You can, for example, set alarms so that the searcher warns you when something new related to certain search is published, among many other possibilities. But that’s another story…

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